Immersible thermal mass flow meter

ABSTRACT

A thermal anemometer or mass flow meter having temperature and flow velocity sensor elements is provided in which a thin film temperature sensor is provided in the heated sensor of the fluid velocity sensor element of the system. The thin-film sensor is captured at least partially within a spacer or interface member, the spacer being received within a housing. The thermal anemometer is constructed to offer sufficient precision and accuracy in its design to be suitable for sensitive scientific and industrial applications. This goal is achieved while using cost effective parts—as in the thin film temperature sensor(s)—in connection with a construction approach minimizing or eliminating gaps or other system configuration variability.

FIELD OF THE INVENTION

This invention relates to mass flow meters, particularly regarding theirmanufacture at decreased cost, yet of such quality for criticalapplications.

BACKGROUND OF THE INVENTION

The mass flow rate of a fluid (defined by its average velocitymultiplied by its mass density multiplied by the cross-sectional area ofthe channel through which the flow travels) is a measured quantity ofinterest in the control or monitoring of most practical and industrialapplications, such as any chemical reaction, combustion, heating,cooling, drying, mixing, fluid power, etc. Generally speaking, a thermalanemometer is used to measure the mass velocity at a point or small areain a flowing fluid—be it liquid or gas. The mass velocity of a flowingfluid is its velocity referenced to standard or normal temperature andpressure. The mass velocity averaged over the flow channel'scross-sectional area multiplied by the cross-sectional area is thestandard or normal volumetric flow rate through the channel and is acommon way of expressing the total mass flow rate through the channel.

The thermal anemometer is sometimes referred to as an immersible thermalmass flow meter because it is immersed in a flow stream or channel incontrast to other thermal mass flow meter systems, such as those whichsense the total mass flow rate by means of a heated capillary tubemounted externally to the flow channel. The operational principles ofthermal anemometers derives from the fact that a heated sensor placed ina fluid stream transfers heat to the fluid in proportion to the massflow rate of the fluid. In a thermal anemometer, one such heated sensoris provided together with another sensor that detects fluid temperature.In the constant-temperature mode of operation, the heated sensor ismaintained at a constant temperature above the fluid temperature. Thetemperature difference between the flowing fluid and the heated sensorresults in an electrical power demand in maintaining this constanttemperature difference that increases proportional to the fluid massflow rate and that can be calculated. Alternately, some thermalanemometers operate in a constant-current mode wherein a constantcurrent or power is applied to the heated sensor and the fluid mass flowrate is calculated from the difference in the temperature of the heatedsensor and the fluid temperature sensor, which decreases as mass flowrate increases. Thermal anemometers have greater application to gases,rather than liquids, because their sensitivity in gases is higher thanin liquids.

Because the parts of the heated sensor of known thermal anemometers arenot sufficiently reproducible dimensionally or electrically, knownthermal anemometers require multi-point flow calibration of electricaloutput versus mass flow rate, usually in the actual fluid and with theactual ranges of fluid temperature and pressure of the application. Forindustrial applications, the heated sensor and fluid temperature sensorof known thermal anemometers typically have their respective sensorsencased in a protective housing (e.g., thermowell or metallic tubesealed at its end, etc.). Usually, the encased heated sensor is insertedinto the tip of the housing and is surrounded by a potting compound,such as epoxy, ceramic cement, thermal grease, or alumina powder.

In such a system, “skin resistance” and stem conduction are two majorcontributors to non-ideal behavior and measurement errors in thermalanemometers constructed in this manner. Skin resistance is the thermalresistance between the encased heated sensor and the external surface ofthe housing exposed to the fluid flow. The well-known hot-wire thermalanemometers have zero skin resistance, but thermal anemometers with ahousing do have skin resistance. The use of a potting compoundsubstantially increases the skin resistance because such pottingcompounds have a relatively low thermal conductivity.

Skin resistance results in a temperature drop between the encased heatedsensor and the external surface of the housing which increases as theelectrical power supplied to the heated sensor increases. Skinresistance creates a “droop” and decreased sensitivity in the powerversus fluid mass flow rate calibration curve which is difficult toquantify and usually varies from meter to meter because of variationsboth in the parts of construction and in installation. The ultimateresult of these skin-resistance problems is reduced accuracy.Furthermore, the use of a surrounding potting compound can createlong-term measurement errors caused by aging and by cracking due todifferential thermal expansion between the parts of the heated sensor.

Stem conduction causes a fraction of the electrical power supplied tothe encased heated sensor to be passed through the stem of the heatedsensor, down the housing, lead wires, and other internal parts of theheated sensor, and ultimately to the exterior of the fluid flow channel.Stem conduction couples the electrical power supplied to the encasedheated sensor to the ambient temperature outside the channel. If theambient temperature changes, stem conduction changes, and measurementerrors occur. Similarly, stem conduction is responsible for errors inthe encased fluid temperature sensor's measurement because it too iscoupled to the ambient temperature.

Further discussion of the operational principles of known immersiblethermal mass flow meters, their several configurations, particularadvantages, uses, skin resistance, and stem conduction are presented insection 29.2 entitled “Thermal Anemometry” by the inventor hereof aspresented in The Measurement Instrumentation and Sensors Handbook, aswell as U.S. Pat. Nos. 5,880,365; 5,879,082; and 5,780,736, all assignedto Sierra Instruments, Inc., and each incorporated by reference hereinin its entirety.

As noted in the referenced material, resistance temperature detectors(RTDs) may be employed in the heated sensor and the fluid temperaturesensor, when one is provided. Alternative sensors for either the heatedsensor or the fluid temperature sensor include thermocouples,thermopiles, thermistors, and semiconductor junction thermometers. RTDsensors are generally recognized as being more accurate and stable thanany of these alternatives.

RTD sensors operate on the principle of electrical resistance increasingin accordance with increasing temperature. In known thermal anemometers,the RTDs are provided most commonly in the form of wire-wound sensors,but also as thin-film sensors (such as provided on an alumina chip) andleast commonly as micro-machined sensors (such as provided in a siliconwafer). The most common wire-wound RTD sensors are usually manufacturedvia hand winding and hand resistance trimming, as well as other manualoperations. This makes them vulnerable to human error in production andsubject to irreproducibilities. The labor content, as well as the highcost of platinum wire, make them quite costly. Variations in thedimensions of the circular mandrel (e.g., alumina) over which the wireis typically wound and the insulating coating (e.g., glass) over thewound wires cause further dimensional and electrical-resistanceirreproducibilities in wire-wound RTD sensors. Micro-machined RTDsensors have even worse dimensional and electrical resistancetolerances. As such, neither type of sensor is ideal for use in thermalanemometers.

On the other hand, thin-film RTD sensors are mass produced usingautomated production operations, employing technologies such asphotolithography and lasers. This results in the comparatively highreproducibility, accuracy, stability, and cost-effectiveness ofthin-film RTD sensors. Yet, prior to the teaching offered by the presentinvention, some thermal anemonmeters have used thin-film RTD's that werenot entirely encased in a protective housing and which had theirsurfaces directly exposed to the fluid. Due to the fragility, poorerdimensional tolerances, and the oscillating and turbulent flow aroundthe thin-film RTD body, etc., such devices—standing alone—have onlyproven suitable for light duty, low-end, low-accuracy/precisionrequirement applications.

Prior to the solution offered by the present invention, the bestaccuracy typically achievable in current thermal anemometers forindustrial applications was approximately 2% to 3% of reading error inaccuracy over a mass flow rate range of 10% to 100% of full scale andover a relatively smaller temperature and pressure range. Theconstruction of the heated sensor selected is what limits the accuracy.Most commonly, a wire-wound RTD sensor and, less commonly, a thin-filmRTD sensor is encased in the tip of a metallic tube (e.g., 316 stainlesssteel) sealed at its end and surrounded by a potting compound (e.g.,epoxy, ceramic cement, thermal grease, or alumina powder).

Sensor fabrication with such potting compounds is inherentlyirreproducible due to variations in their composition, amount used,insufficient wetting of surfaces, and/or air bubbles. In the case ofwire-wound sensors, this irreproducibility is added to previouslymentioned irreproducibilities associated with wire-wound RTD sensorsthemselves. These irreproducibilities, combined with the previouslymentioned high skin resistance and potential for long-term instabilityassociated with the use of potting compounds, limits the overallaccuracy of known thermal anemometers constructed in this manner.

The thermal anemometer described in U.S. Pat. No. 5,880,365 avoids theaccuracy degrading use of potting compounds by forming the encasinghousing over the wire-wound RTD sensor by means of forces external tothe housing. This construction has high stability and improved accuracybut is relatively expensive and may have irreproducibilities associatedboth with wire-wound RTD sensors and with variations from meter to meterin the gap between the wire-wound RTD and the internal surface of thehousing.

However, the present invention employs a thin-film RTD not prone to suchproblems. It does so in a manner not heretofore contemplated, therebyoffering the advantage of the sensor type's relative benefits, but in ahighly accurate meter. As such, the present invention offers asignificant advance in the art.

SUMMARY OF THE INVENTION

Where a thermal anemometer is desired for use in a given application,the quality of the device may be quite significant. The presentinvention offers a mode of device construction or packaging inconnection with thin-film RTD sensors (TFRTD) that is able to leveragethe cost advantage offered by such products, but still attains andimproves the measurement quality required of scientific and industrialapplications. Namely, systems according to the present invention offerperformance with as low as 1% to 1.5% or 2% of reading error in accuracyover a mass flow rate range of 10% to 100% of full scale (or larger) andover a relatively larger fluid temperature and pressure range.

When coupled with computations based on heat-transfer correlations andother corrective algorithms, reducing the dimensional and electrical(e.g., resistance) tolerances of the parts of the heated sensor as ispossible with the present invention yields important cost-reducing andaccuracy-enhancing benefits. These potential benefits include: fewerflow calibration points required; calibration with a low-cost surrogateflow calibration fluid (e.g., air for other gases); and better accuracyover wider ranges for mass flow rate, fluid temperature, and fluidpressure. In the ultimate case of negligible tolerances (i.e., perfectreproducibility), no flow calibration whatsoever is required.

To achieve one or more of these benefits, the present invention providesan approach for using a TFRTD temperature sensor(s) in an immersiblethermal mass flow meter. The meter may be configured in connection withrelevant hardware for use as an insertion or as an in-line type device.The meters include temperature and velocity sensor elements. Thevelocity sensor element has a heated TFRTD sensor and may also include asecondary temperature sensor to enable compensation for stem conduction.The temperature sensor element may also include a second temperaturesensor for stem-conduction compensation as described in U.S. Pat. No.5,879,082. The meter's sensor elements are typically used in connectionwith a programmed general-purpose computer or dedicated electroniccontrol hardware—either example of such hardware including a dataprocessor.

In each variation of the invention, the heated sensor in the velocitysensor element is a TFRTD sensor. Although the preferred embodiment ofthe present invention uses TFRTDs for the remaining temperature sensorsin both the velocity sensor element and the temperature sensor element,alternative types of temperature sensors can be used in these locations.

Means of producing the heated TFRTD of the velocity sensor elementinclude printed circuit technology, photolithography, laser milling, andMEMs approaches, etc., whereby a temperature sensing element is providedon a silicon wafer, by thin-film platinum, nickel, or other metal on analumina or other electrically insulating chip, or otherwise. By virtueof the manner in which the heated TFRTD is held or captured within thevelocity sensor element, spacing or gaps between it and adjacentthermally conductive material are minimized or effectively eliminated.

In the subject heated TFRTD of the velocity sensor element of mass flowmeters according to the present invention, an outer layer is provided bya housing that captures a spacer or interface member (gland), that—inturn—captures the TFRTD. During manufacture, gaps between the TFRTD andthe spacer adapted to receive the same are minimized or eliminated usinga method in which the ductile metal forming the spacer is compressedaround or about the TFRTD. Such an approach may take place prior toinsertion or encasement of the spacer into an outer housing (e.g.,thermowell or tube).

Beyond selecting a ductile material for the spacer, certain othermaterial-choice considerations, in any combination, may be consideredpertinent. For one, the spacer material may have a high thermalconductivity in order to minimize skin resistance and to provide a moreuniform axial temperature distribution along the length of the spacer,thereby simplifying the use of heat-transfer correlations and othercorrective algorithms for the velocity sensor element. For another, itmay be desirable to produce the spacer from powdered metal for the sakeof economy in producing the desired shape. Alternatively, the spacermaterial may be selected in coordination with that of the housing andTRFTD in order to match or substantially match thermal expansionproperties.

In any case, the approach to heated TFRTD capture within the tightesttolerances using an interface member is a major reason why the velocitysensor element—and thus—thermal anemometers according to the inventionoffer the requisite accuracy and precision for industrial and scientificapplications. Yet, it is the inexpensive nature, reproducible dimensionsand electrical characteristics of using a thin-film RTD temperaturesensor (at least as the heated sensor of the velocity sensor element)that make systems according to the present invention economical.

The tangible benefits to users of immersible thermal mass flow metersaccording to the present invention may include reduced cost, higheraccuracy, and higher stability than know alternatives The features ofthe invention yield these benefits by solving the problems of knownthermal anemometers described in the Background section above asfollows: the use of a TFRTD as the heater of the velocity sensor elementreduces cost and enhances reproducibility issues; the high thermalconductivity interface member assembled with minimal gaps reduces skinresistance and simplifies the use of heat-transfer correlations andcorrective algorithms; the “dry” sub-assembly of the heated sensor ofthe velocity sensor element fabricated without the use of pottingcompounds increases long-term stability and reproducibility; anadditional optional temperature sensor in the velocity sensor elementand/or temperature sensor element compensates for stem conduction. Inthe above features improved reproducibility yields the benefit ofreduced costs; simplified heat-transfer correlations and correctivealgorithms yield the benefits of better accuracy and reduced cost; andreduced skin resistance and stem-conduction compensation both yield thebenefit of better accuracy.

In sum, the present invention includes systems comprising any of thefeatures described herein. As for these features and possible advantagesenjoyed in connection therewith, only the use of at least one TFRTD isrequired in the invention. All other advantageous aspects are optional.Methodology, especially in connection with manufacture, also forms partof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Each of the figures diagrammatically illustrates aspects of theinvention. Of these:

FIGS. 1A and 1B show front and side views, respectively, of a thin-filmRTD (TFRTD) sensor element;

FIGS. 2A and 2B show an end view and a side sectional view taken alongline A-A of a spacer interface member for the temperature sensor shownin FIGS. 1A and 1B;

FIG. 3 is a partial side-sectional view of a velocity sensor element inaccordance with the present invention incorporating the hardware shownin FIGS. 1A-2B;

FIGS. 4A and 4B show front and side views, respectively, of anothertemperature sensor as may be used in the present invention;

FIG. 5 shows a partial sectional view of the velocity sensor elementincluding the sensor in FIGS. 4A and 4B housed therein to providecompensation for stem conduction;

FIG. 6 shows a partial side-sectional view of a complete thermalanemometer sensor assembly including velocity and temperature sensorelements according to the present invention;

FIG. 7 shows a partial sectional view of a preferred temperature sensorelement;

FIG. 8 shows a partial side-sectional view of a sensor head of a thermalanemometer with an insertion-type configuration according to the presentinvention with the assembly of FIG. 6 set therein;

FIGS. 9A and 9B show a partial side-sectional view and an end view,respectively, of a thermal anemometer according to the present inventionof the insertion-type configuration having a tubular stem;

FIG. 10 shows a partial side-sectional view of an alternativeinsertion-type meter configuration;

FIGS. 11A and 11B show an end view and a side-sectional view taken alongline B-B, respectively, of a thermal mass flow meter according to thepresent invention of the in-line-type configuration; and

FIGS. 12A and 12B show partial side-sectional views of two alternativein-line-type configurations.

Variation of the invention from that shown in the figures iscontemplated. Fluid flow direction is indicated in many of the figuresby arrows.

DETAILED DESCRIPTION

Before the present invention is described in detail, it is to beunderstood that this invention is not limited to particular variationsset forth and may, of course, vary. Various changes may be made to theinvention described and equivalents may be substituted without departingfrom the true spirit and scope of the invention. In addition, manymodifications may be made to adapt a particular situation, material,composition of matter, process, process act(s) or step(s), to theobjective(s), spirit or scope of the present invention. All suchmodifications are intended to be within the scope of the claims madeherein.

Methods recited herein may be carried out in any order of the recitedevents which is logically possible, as well as the recited order ofevents. Furthermore, where a range of values is provided, it isunderstood that every intervening value, between the upper and lowerlimit of that range and any other stated or intervening value in thestated range is encompassed within the invention. Also, it iscontemplated that any optional feature of the inventive variationsdescribed may be set forth and claimed independently, or in combinationwith any one or more of the features described herein.

All existing subject matter mentioned herein (e.g., publications,patents, patent applications and hardware) is incorporated by referenceherein in its entirety except insofar as the subject matter may conflictwith that of the present invention (in which case what is present hereinshall prevail). The referenced items are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such material by virtue of prior invention.

Reference to a singular item, includes the possibility that there areplural of the same items present. More specifically, as used herein andin the appended claims, the singular forms “a,” “and,” “said,” and “the”include plural referents unless the context clearly dictates otherwise.It is further noted that the claims may be drafted to exclude anyoptional element. As such, this statement is intended to serve asantecedent basis for use of such exclusive terminology as “solely,”“only” and the like in connection with the recitation of claim elements,or use of a “negative” limitation. Unless defined otherwise herein, alltechnical and scientific terms used herein have the same meaning ascommonly understood by one of ordinary skill in the art to which thisinvention belongs.

Turning now to FIGS. 1A and 1B, these show a view of the type oftemperature sensor 2 employed in the present invention. The sensor shownis a “thin film” type sensor as described above. The particular sensorshown is a thin-film Platinum Resistance Temperature Detector (PRTD) ascommonly available. An active region 12 of the device is provided, overwhich area the PRTD is self-heated by current during use. Sensor 2includes lead wires 4 connected to weld pads leading to active region 12and covered by a glass strain relief 10. The body 8 of the sensor ismade of high-purity alumina, preferably held to a thickness tolerancewithin about ±0.002 to 0.001 inches as commonly available. A thin layerof glass electrical insulation is provided over the PRTD active area. Ofcourse, the PRTD is only exemplary as other such TFRTDs may be employedin the invention.

FIGS. 2A and 2B, show a member for receiving the RTD therein. Theinterface member or spacer 20 comprises a metal such as copper. Thematerial should be highly thermally conductive. For example, othermetals and alloys (such as bronze) including aluminum, aluminum alloy,silver, gold, alloys thereof, etc. could be employed.

Space 20 is advantageously constructed of molded powdered metal. In thatmanner it is cost-effectively constructed to define a hole or bore 22suited to closely fit temperature sensor 2. Other techniques, such asbroaching and electric discharge machining, may be employed in producingthe part. However, as a molded powdered metal piece, the item isinexpensive to produce.

Still further, as a powdered metal piece, the inventor hereof observedan unexpected result. Namely, when the sensor was inserted into thehole, any gap between the sensor and the bore is permanently collapsedby squeezing the exterior of the spacer in a simple chuck. Such ease ofplastic deformation of the piece is believed (at least in part) toresult from the nature of the powdered metal matrix.

Yet, whether the spacer is a powdered metal piece or otherwise formed,more aggressive forming techniques can be applied to reduce any gapsbetween the sensor and facing material. Particularly, a hydroformingprocedure such as described in the '365 patent referenced above may beemployed. It should be noted that such forming techniques should notapply forces on sensor 2 sufficient to generate electrical resistancechanges caused by strains in sensor number 2. Our tests have shown thatwith simple mechanical compression of a powdered metal copper spacer andrelease from the compression, sensor 2 is not easily pressed from hole20 (when opened at a distal end 24 to allow such action).

Accordingly, it has been surmised that one or more points or sub-areasof the two larger-area surfaces 28 of the hole 22 in spacer 20 aretouching mating points or sub-areas of the two larger-area surfaces 14of temperature sensor 2, thereby clamping temperature sensor 2, withforces substantially normal to the sensor, in an immovable positionrelative to the spacer 20 and insuring long-term stability of thesubassembly. The gaps between the two smaller-area surfaces of the hole22 and the two smaller-area surfaces of the temperature sensor 2 may bedesigned to be relatively larger and thereby have little effect in thecompression process.

It is further surmised, based on material variations and testing, thatthe total gap—the sum of the gap between one surface 28 of the hole 22and its mating surface 14 of the temperature sensor 2 and the gapbetween the second pair of such surfaces—ranges from about 0.003 inchesto 0.0005 inches (and where mating surfaces touch even smaller) andlikely averages about 0.001 inches. That is to say, the sensor thickness“ST” and gap width “GW” as shown in FIGS. 1B and 2B respectively differby between about 0.0005 and 0.003 inches, or on average about 0.001 toabout 0.002 inches. Yet another way to view the interaction is in termsof a resulting close fit or light press fit tolerances as commonlyunderstood by those with skill in the art.

In the present invention, spacer 20 provides an intermediate memberbetween sensor 2 and the shell housing 32 of the velocity sensor element30. In the '365 patent, it is the housing that is externally pressureformed. In the present invention, no such activity occurs in connectionwith the housing. Instead, spacer 20 (together with sensor 2) ispress-fit into the housing. The rounded distal end 24 of spacer 20 ispressed into a complimentary distal section 34 of housing 32. In thismanner, gaps along the outer surface of the bullet-shaped spacersubassembly are substantially eliminated.

The fit between the spacer and its housing may be designed as a closefit, a light press fit or even a heavy press fit. In the latter cases,the housing exerts a radial force on the spacer subassembly. This forcemay supplement any previously-applied compression to the spacer 20 ontotemperature sensor 2 or it may instead offer the only compression ontotemperature sensor 2 applied in some cases.

Seeing as a goal of the spacer is to eliminate gaps (which are more thanan order of magnitude less thermally conductive than the desired metalmaterial) that increase skin resistance and thereby decrease systemaccuracy and introduce variability interfering with modeling of thesystem, the shape of bore 22 will depend on the shape of the sensor.Regardless of the sensor shape, it is preferred that at least theportion 26 of the spacer facing the active surface 12 of the RTD beshaped to facilitate direct and at least substantially complete abutmentof the surfaces.

Still further, it is to be appreciated that the inventive system is onein which no significant amount of fillers or potting material such asepoxy, ceramic cement, thermal grease, alumina powder, or another agentor compound is provided between sensor 2, spacer 20, and housing 32 ofvelocity sensor element 30. This factor is important since introductionof such material introduces in quantity (or at all) may introduceirreproducibilities in the velocity sensor element. It may furtherintroduce instability, for example, by virtue of differential expansionduring temperature cycling. Still, one might add a very thin layer ofthermally conductive material, otherwise used as “potting compound” toall gaps in effort to reduce skin resistance between elements. However,such a film, veneer or wetting of components (likely performed prior totheir assembly) is different, in kind, to immersing or traditionally“potting” an item in such material.

On another front, due to thermal cycling it is also desirable that atleast spacer 20 and housing 32 have substantially or approximatelymatched coefficients of thermal expansion. Such a result is possible inthe case of a pairing of copper and stainless steel for the spacer andhousing, respectively. Of course, other materials may be selected. Yet,the copper/stainless combination has proven highly advantageous byvirtue of the ductility of the copper in the forming procedure of thespacer against sensor 2 and further with respect to the good materialthermal expansion rate match the selection provides.

As for further optional features of the invention, reference is made toFIGS. 4A and 4B. Here, a second RTD 40 is shown. Like that shown inFIGS. 1A and 1B, it includes leads 42, a substrate 44, strain relief 46,and active area 48. Yet, this sensor is not self-heated. It is intendedmerely to measure temperature. When optionally used in the velocitysensor element 30, its purpose is to offer compensation for stemconduction as is known possible in the art as possible in theory.However, the hardware implementation offering potential for suchcalculations is unique to the present invention.

In addition to such other facets as one with skill in the art willappreciate upon review of the present disclosure, FIG. 5 illustrates thedesired placement of sensor 40 within the body of velocity sensorelement 30, in which the adjacent housing 32 of velocity sensor element30 is in the full fluid flow stream substantially identical to that ofheated sensor 2. The use of sensor 40 to compensate for stem conductionis greatly simplified if the distal length of the velocity sensorelement 30 from its far distal tip to the active area of sensor 40,called the “active length,” is in the full fluid flow with a velocityprofile over said active length that is substantially uniform. Thesesame considerations also are applicable to the location of the secondtemperature sensor 70 in FIG. 7 for stem conduction compensation in thetemperature sensor element 56. Nevertheless, the present inventionencompasses, but with reduced accuracy for stem conduction compensation,the placement of sensor 40 at any location within the velocity sensorelement, including more proximal locations, including within the cavitynoted by location 81 in FIG. 8 or even at the base of the velocitysensor element within sensor head 80 as noted by location 83 in FIG. 8.Yet, in a preferred variation of the invention, the housing 32 ofvelocity sensor element 30 that is adjacent to sensor 40 is in the fullflow stream because then the computations associated with determiningmass flow are comparably simple or elegant.

FIG. 5 offers a cross-sectional view of the highlighted section of thevelocity sensor element in FIG. 6. In this sectional view, the cavity inferrule piece 50 into which sensor 40 is fit is shown filled with thesensor. For the sole purpose of improving the thermal contact betweensensor 40, ferrule 50, and housing 32, a potting compound such asthermally conductive epoxy or ceramic cement may surround sensor 40 inthe cavity of ferrule 50. It should be observed that said pottingcompound is outside of the velocity sensing length of the velocitysensor element, and therefore any of its previously described negativefeatures do not affect accuracy or long-term stability. In FIG. 3, nosuch sensor is provided, illustrating the optional nature of the sensor.

While including the sensor 40 offers certain advantages in the abilityto broadly provide compensation for stem conduction, it still may bedesired to provide a longitudinal spacer collar 54 to carefully definethe distance between ferrule 50 (for when it might carry a sensor 40)and spacer 20 which carries sensor 2.

As for the more global construction of a thermal mass flow meteraccording to the present invention, FIG. 6 illustrates the velocitysensor element/assembly 30 and temperature sensor element/assembly 56provided in a greater sensor housing assembly 60. The sensor elementassemblies are set within sensor head 62 with their respective leadsoptionally potted in epoxy, cement (or the like) with insulated wires 64arranged for connection to a processor 66.

While such constructional details are within the level of those withskill in the art to handle without undue experimentation, FIG. 7illustrates a particular temperature sensor element 56 as advantageouslyemployed in the present invention. As illustrated, the assemblypreferably includes two TFRTDs as shown in FIGS. 4A and 4B. The distalsensor 72 is the primary sensor for measuring the temperature of theflowing fluid. The proximal sensor 70 compensates for stem conduction asdescribed in U.S. Pat. No. 5,879,082. In some applications, such asthose involving certain liquids and certain gases at high velocity,stem-conduction errors are relatively small and in those applicationsproximal temperature sensor 70 is not needed. It is understood thatproximal temperature sensor 70 is optional to, and is not required by,the present invention. It is further understood that, although TFRTDsare the preferred type of temperature sensor for use as sensors 70 and72, the present invention encompasses the use, in any combination, ofother types of temperature sensors such as wire-wound RTDs,thermocouples, and thermistors.

Yet another advantageous innovation that may be desired for use inconnection with the present invention for thermal anemometers of theinsertion configuration is shown in FIG. 8. Here an open-endedprotective sensor head 80 is shown in partial cross section. Thesectional view reveals the placement of the velocity and temperaturesensor elements in the sensor head. On either side of the sensorelements/assemblies, legs 82 defining an open channel and extendingbeyond the sensor elements are provided. The legs are of particular usewhen a technician is installing a completed meter into a pipe section orother location. The legs prevent inadvertent damage of the sensorelements during the installation procedure as well as offeringprotection from mishandling in the meantime. Use of a protective shieldfor the sensor elements of insertion thermal anemometers has precedence,but such shields normally are closed at their distal end. The shieldingof sensor head 80 of the present invention is open at its end andthereby eliminates the flow disturbance created at the distal end ofclosed ended shields and consummates ultimately in better accuracy.

FIGS. 9A and 9B show a complete probe assembly of an insertion meter ofthe present invention constructed with tubular stem 88 and the sensorhead 80 of FIG. 8. This meter is sealed and connected to the flowchannel or stream by means of a compressing fitting, flange or otherlike means. The constituent elements of the system are as described anddesignated by numerals above. To facilitate proper installationorientation by an end-user a pointer indicating flow direction may beincorporated in the housing.

FIG. 10 shows another insertion thermal anemometer configuration of thepresent invention intended for applications not requiring the highestaccuracy. In contrast to the insertion meter of FIGS. 9A and 9B, thismeter has threaded process connection 32 and, for purposes of strength,a closed-ended protective shield 80′ around the sensor elements.

Whereas the largest portion of the flow of a fluid around such knownthermal anemometers of the insertion configuration flowscircumferentially and perpendicularly around the meter'sperpendicularly-oriented stem 88 and sensor head 80 of FIGS. 9A and 9B(head detailed in FIG. 8), nevertheless a smaller fraction is inclinedto flow axially down the stem and sensor head, enter the volume betweenlegs 82, and ultimately flow over the velocity sensor element 30. Thiseffect can cause the meter to erroneously measure a velocity higher thanthe actual velocity. Since this axial flow varies with the depth ofinsertion into the flow stream, its magnitude during flow calibrationmay be different than that of the actual field application, therebyimpairing velocity measurement accuracy.

Accordingly, insertion meter sensor head 80 of the present invention isdesigned to reduce errors caused by such axial flows. Shoulder 84 andinset 86 of sensor head 80 in FIG. 8 provide aerodynamic features toredirect and divert said axial flow circumferentially around the sensorhead as indicated by the flow arrows, thereby diminishing its magnitudepassing over the velocity sensor element 30 and improving velocitymeasurement accuracy. Furthermore, one with skill in the art ofaerodynamics will recognize that one or more shoulders 84, fins, orother alternate feature configurations may be provided to redirect aportion of said axial flow circumferentially and substantiallyperpendicularly around the insertion probe before it passes over thevelocity sensor element and causes errors. The present inventionencompasses the use of such elements intended for the purpose stated andassociated flow dynamic methodology.

Turning to FIGS. 11A and 11B, these drawings illustrate an immersiblethermal mass flow meter of the in-line configuration encompassed by thepresent invention. The mass flow meter assembly 90 is shown emplacedwithin an adapter 92 extending from pipe 94. Because the velocity sensorelement 30 and the temperature sensor element 56 are intended to beenclosed within the pipe 94 as a delivered unit for in-line placementwithin a system, the sensors require no protective shield. In-line meter90 is attached to the process piping by means of flanges 98. Alsopictured are two perforated flow plates 96 in series and upstream ofvelocity sensor element 30 and temperature sensor element 56 in orderthat the flow reaching the same may be substantially uniform andindependent of upstream pipe disturbances.

In most known insertion-type and in-line configuration immersiblethermal mass flow meters the velocity sensor and temperature sensorelements are aligned substantially perpendicular to the main fluid flowstream as shown in FIGS. 11A and 11B (and as indicated by thedouble-line flow direction arrows in many of the preceding figures).However, the in-line meters 100 and 110 in FIGS. 12A and 12B,respectively, represent exceptions to this commonality and have theirflow axial to the sensor elements. These designs are designed primarilyfor applications with low mass flow rates and therefore have relativelysmall flow channels.

Accordingly, meter 100 in FIG. 12A has flow channels machined in flowbody 102 which connect to manifold 106. Meter 110 in FIG. 12B hastubular flow channels 112 connected to manifold 104. Meter 100 haspipe-threaded process connections 104, and meter 110 has tubular processconnections 112. In both of these configurations, the fluid flowsaxially over the sensor elements, as described in U.S. Pat. No.5,780,776, rather than perpendicular to the sensors as in the meterspreviously described.

The present invention is suited for use in connection with various otherflow meter configurations in addition to those shown the variousfigures. As for other manners in which the present invention may beimplemented (i.e., housed or integrated in a flow system), these areeither known or readily appreciated by one with skill in the art;further examples of which are sold by Sierra Instruments, Inc.

The thermal anemometer of the invention retains advantageous performanceif operated with either digital or analog sensor-drive electronics, orwith a combination of both, in either the constant-temperature orconstant-current modes of operations, all as described in the abovementioned book chapter authored by the inventor hereof. Digitalelectronics may be preferred for reason of simplified computations basedon heat-transfer correlations and corrective algorithms, that compensatefor any changes (e.g., as referenced to flow calibration conditions) inthe fluid itself, fluid temperature, fluid pressure, ambienttemperature, and other variables and influence parameters, therebyyielding higher system accuracy. Said heat-transfer correlations andcorrective algorithms are based on known empirical heat transfercorrelations, specific experimental data for the thermal anemometer ofthe present invention, physics-based heat transfer theory, and othersources.

Though the invention has been described in reference to severalexamples, optionally incorporating various features, the invention isnot to be limited to that which is described or indicated ascontemplated with respect to each embodiment or variation of theinvention. The breadth of the present invention is to be limited only bythe literal or equitable scope of the following claims.

1. An apparatus for use in a mass flow meter for immersion in a fluid,comprising: a velocity sensor element comprising an elongate body forextending into the fluid, said elongate body comprising a housing shell,a distal end of said housing shell receiving and closely holding aspacer therein, said spacer comprising a solid body of metal receivingand closely holding a thin-film Resistance Temperature Detector (RTD)sensor therein, said sensor comprising an active area and electricalleads to carry current to said active area from a proximal end of saidshell, said active area in substantially gap-free contact with aninternal abutting spacer area.
 2. The apparatus of claim 1, furthercomprising a second temperature sensor.
 3. The apparatus of claim 2,wherein said second temperature sensor is a thin-film RTD sensor.
 4. Theapparatus of claim 1, wherein said spacer is in substantially gap-freecontact with said housing shell.
 5. The apparatus of claim 1, furthercomprising a temperature sensor element comprising a fluid temperaturesensor.
 6. The apparatus of claim 5, wherein said fluid temperaturesensor element comprises two temperature sensors within a housing. 7.The apparatus of claim 6, wherein said temperature sensors are thin-filmRTD temperature sensors.
 8. The apparatus of claim 5, further comprisinga computer processor.
 9. The apparatus of claim 5, further comprising anopen protective housing adapted to axially receive said velocity andtemperature sensor elements.
 10. The apparatus of claim 9, wherein saidopen protective housing includes at least one feature proximal to adistal end of either sensor element at the exterior of the housing toredirect the axial component of the velocity vector of the fluid flowingover the exterior of the housing.
 11. The apparatus of claim 9, whereina distal end of said housing is closed.
 12. The apparatus of claim 10,wherein said feature comprises a shoulder section.
 13. The apparatus ofclaim 5, configured as an insertion flow meter.
 14. The apparatus ofclaim 5, configured as an in-line flow meter.
 15. The apparatus of claim1, wherein said spacer comprises a powdered metal fabricated piece. 16.The apparatus of claim 15, wherein said powdered metal comprises copper.17. The apparatus of claim 15, wherein said housing shell comprisesstainless steel.
 18. The apparatus of claim 1, wherein said housingshell exerts radial force upon said spacer and said spacer exerts forceholding said temperature sensor in stable position. 19.-32. (canceled)